Cannulas can be used to deliver materials into the central nervous system (CNS) of a subject. However, with current cannula designs, care must be taken to prevent reflux of the material along the injection track. Quereshi et al. (2000) Neurosurgery 46(3): 663-69. Even with precautions are taken to minimize reflux, such as slow removal of the cannula and the application of pressure to the tissue as the cannula is removed, reflux remains a problem.
In addition, a substantial portion of the material being delivered can be lost due to exposure of the material to the large surface area of the inside of the cannula. In particular, exposure to stainless steel can cause substantial loss of the material to be delivered. For example, various groups have demonstrated that a substantial amount of adenovirus vectors preparations exposed to stainless steel surfaces are lost. Naimark et al. (2003) Hum. Gene Ther. 14:161-6; Tsui et al. (2001) Mol. Ther. 3:122-5; Marshall et al. (2000) Mol. Ther. 1(5 Pt 1):423-9. The problem is exacerbated when very small volumes of material are being delivered, because the smaller the volume, the greater the ratio of surface area to volume within the cannula. Given that the use of small volumes of material is particularly desirable in situations where the material is expensive or difficult to obtain, it would be desirable to have devices and methods in which both reflux and loss of material are minimized.
Thus, there exists a need for a cannula capable of introducing materials into the brain of a subject without reflux of the material along the needle track. A need also exists for cannula designs that reduce the loss of agents to the inner surface(s), and, accordingly, can deliver small volumes of material effectively.